In vehicles with manually shifted transmissions it is possible by engaging a gear, in particular the first gear, to secure the vehicle when parked and with its engine stopped against rolling away, since when a gear ratio is engaged, a force link is formed between the drive wheels and the stationary crankshaft of the internal combustion engine. In automatic transmissions the force link when a gear is engaged in the transmission is formed by applying pressure to shifting elements. When a vehicle is parked, since the engine is stopped the pump of the automatic transmission is not being driven, so no actuating pressure can be built up. Accordingly, in the parked condition no force link can be formed between the drive wheels and the internal combustion engine. To secure a vehicle with an automatic transmission against rolling away a so-termed parking lock is arranged in the automatic transmission, which is designed as an interlocking, shiftable device for locking the output side of the transmission or the drive-train to the drive wheels.
The interlocking coupling elements of the parking lock are actuated by means of a piston of a hydraulic parking lock cylinder, which is controlled by a hydraulic shifting device of the transmission control system.
The disclosure document DE 102007050802A1 describes a transmission control system for an automatic transmission with an electro-hydraulic shifting device for controlling a parking lock. The electro-hydraulic shifting device in this case comprises a parking lock valve in the form of a 2/2-way valve with a valve slide and a compression spring, such that depending on the shift position of the valve slide, the parking lock cylinder is connected to a pressure oil supply held at a main pressure, or cut off from the pressure oil supply. When the parking lock cylinder is connected to the pressure oil supply a pressure chamber in the parking lock cylinder is acted upon by the main pressure, and the resulting displacement of a parking lock piston actuates the parking lock. Thus, in the parking lock cylinder the main pressure acts as the working pressure since in the parking lock cylinder hydraulic energy is converted into mechanical energy. In this case the pressure oil supply acts as the source of the working pressure. When the parking lock cylinder is acted upon by pressure, the interlocked connection of the parking lock is released, also said to be “disengaged”. Analogously, the parking lock is “engaged” when the interlocked connection is in place and the output side of the transmission is locked.
The parking lock valve is connected to various pressure outlets of the transmission control system, denoted hereinafter as control pressure sources, and is controlled by their shifting or regulated pressures, also denoted as control pressures. These control pressures are understood to be pressures that actuate a valve or move a shifting element such as a valve slide to particular shift positions in which an operating medium from a working pressure source is delivered to a hydraulic consumer or cut off therefrom. In this case the valve slide of the parking lock valve is moved in opposition to the force of a spring, from a first shift position in which the pressure oil supply is cut off from the parking lock cylinder, to a second shift position in which the parking lock cylinder is acted upon by the main pressure.
The shifting of the valve slide of the parking lock valve in accordance with the prior art disclosed in DE 10 2007 050 802 A1 takes place by means of two electro-hydraulically adjustable control pressures which can act individually or in combination. In this case the parking lock valve has two pressure chambers, such that in a first pressure chamber, pressure from an electromagnetic pressure regulator can act upon a round axial surface with a circular cross-section. In this context the pressure from the electromagnetic pressure regulator is again referred to as the control pressure and the pressure regulator itself as the control pressure source. In the second pressure chamber an actuating pressure of a shifting element can act upon an annular axial surface of the valve slide. When the pressure chambers are pressurized, the valve slide is moved in opposition to the spring force.
In the selector lever position “P”, which is set when a vehicle is parked, even if the engine is running and the pump is therefore delivering so that the main pressure is present, the two pressure branches are not pressurized, i.e. the two control pressures are virtually equal to zero, so the parking lock remains engaged. If as the result of a fault in the electromagnetic pressure regulators or in the electronic transmission control system that controls them at least one of the two control pressures is built up, the parking lock is disengaged and the vehicle can roll away. For that reason, in present-day systems the driver, when starting the engine, has to actuate the vehicle brakes in order to prevent unintentional rolling away of the vehicle.
In the future, in various markets a so-termed “Remote Start Function” will come into use, with which the driver will be able by operating a remote control device to start the engine in order to switch certain systems to a state of readiness for operation. With this function the previous safety strategy is disadvantageously no longer possible since the driver would not be in the vehicle if a fault as described above led to disengagement of the parking lock unless this had been diagnosed when the engine was stopped and the parking lock engaged. Thus, the vehicle could roll away without the driver being able to prevent this.